1. Field of the Invention
The present invention relates to solvent resistant polymers for use in printing plates, and printing plates employing such polymers. In particular, the present invention relates to a polymer having a monomeric unit comprising a cyclic terminal urea group, which renders the polymer bakeable, and a printing plate employing such a polymer.
2. Background Information
Recently, the resistance of offset printing plates to solvents and common printing room chemicals such as plate cleaning agents or rubber blanket washing agents as well as to alcohol substitutes in the fountain water has had to meet ever increasing demands. Particularly in printing processes using UV-curing inks, where rinsing agents with a high content of esters, ethers or ketones are used, the chemical resistance of conventional positive working printing plates is no longer sufficient without special stabilizing processes. To improve the chemical resistance of positive working offset printing plates, three approaches have essentially been discussed:                1) In the most basic case, positive working offset printing plates comprise two components, namely a naphthoquinone diazide (NQD) derivative and a novolak. One possibility is to chemically alter the NQD such that it has an increased chemical and solvent resistance (e.g. U.S. Pat. No. 5,609,983). The disadvantage of this method is that the resistance of a printing plate produced therefrom cannot be increased indefinitely by increasing the NQD content since the NQD content must lie in a certain range to be viable from a practical point of view.        2) Another approach is to thermally stabilize a conventional positive working printing plate (based on novolak and NQD). This can be carried out either by “baking” or by means of the Pulsar™ process. During baking, a completely developed printing form is heated to about 230° C. for a few minutes causing the mostly phenolic binders to cross-link. This results in an extremely high chemical and mechanical resistance. In the Pulsar™ process, the developed printing plate is subjected to a short-time “shock heating” to about 175° C. This results in a slight cross-linking (and thus an increase in the chemical resistance) of the image areas. For the user, the two thermal stabilizing processes entail the important disadvantage that they require somewhat complicated apparatuses and are therefore quite costly.        3) A third option is the change of the binder content of a radiation-sensitive composition. For example, additives can be added to the formulation. However, in the case of commercially available additives, the amount to be added is often limited since these substances are not adapted to the composition and therefore often affect the properties of the printing plate when added at too high a concentration.        
Another frequently used method is the use of higher molecular novolaks; however, this leads to a deterioration of the photosensitivity of the composition which leads to a considerable loss of time for the user, particularly in the case of high numbers of prints or when so-called “Step & Repeat” exposure machines are used.
Furthermore, the properties of the binders in the photosensitive composition can be improved by way of chemical modification. Such special binders with a high degree of chemical resistance are either not commercially available or very expensive. Accordingly, it is often necessary for manufacturers to develop their own binders. To avoid high costs in the developing process, efforts must be made to keep the synthetic requirements for preparing such tailor-made high-performance binders as simple as possible.
For example, U.S. patent application Ser. No. 09/630,920  U.S. Pat. No. 6,475,692 describes radiation-sensitive compositions which significantly increase the chemical resistance of printed circuit boards for integrated circuits, photomasks and in particular printing forms. However, the compositions of U.S. patent application Ser. No. 09/630,920  U.S. Pat. No. 6,475,692 are not bakeable, which means they do not harden upon heating. The chemical resistance to solvents of the urea-containing copolymers is even higher than for the polymers described in U.S. patent application Ser. No. 09/630,920  U.S. Pat. No. 6,475,692.
JP 90/91752 describes a solvent resistant polymer comprising epoxide functionalities. The polymer is not described as being suitable for thermal applications.
JP 88/121546 describes polymers suitable for UV-ink plates. The polymers contain amide and carboxylic acid groups, but no urea groups.
EP 0 737 896 discloses solvent resistant polymers. The polymers do not contain terminal urea groups and are not described as being bakeable or suitable for thermal applications.
Therefore, despite intensive research in the field of chemically resistant binders for offset printing plates, there is a need in the art for a radiation-sensitive composition with high solvent resistance for use in printing plates. It is an object of this invention to provide such a radiation-sensitive composition. It is another object of this invention to provide a printing plate having such a radiation-sensitive composition.